First of all 1.6 dBi gain is very low try to match the impedance of the antenna ....the reason why you are getting the same response is due to the ground plane. The ground plane will not allow EM wave to pass through it. you can clearly see that in the radiation pattern. the lower hemisphere is consists of minor lobe compare to major lobe in the upper hemisphere that means no radiation along -z axis. In order to understand impact of human body on this antenna you have to create stack of differemt dielectri layers for example skin layer, fat layer, fluid layer etc..